Raster pattern magnetoresistors

ABSTRACT

A thin-film raster pattern magnetoresistor device produced from an electron beam-recrystallized InSb film employing a photolithographic process to etch out of the InSb film a suitable pattern on which micron-size indium or copper stripes are later etched from a superposed metallic film applied on the InSb by vacuum deposition. The thin-film raster pattern magnetoresistor device is a resistor, whose initial resistance in zero magnetic field is between 10 and 1,000 ohms, and whose resistance increases with an applied magnetic field, reaching a value greater than a factor of 10 on the initial resistance in a magnetic field of 10 kOe.

United States Patent [1 1 Collins et al.

[ Dec. 3, 1974 1 RAS-TER PATTERN MAGNETORESISTORS [76] Inventors: 'David A. Collins, 6234 Pearl Lake Ave, Harry Wieder, 2860 Chatsworth Blvd., both of San Diego, Calif,

[22] Filed: Mar. 1, 1971 I 211 Appl. No.1 119,978

Related US. Application Data [63] Continuation-in-part of Ser. No. 748,069, July 26,

1968,Pat. NO. 3,592,708,

52 us. Cl 117/212, 117/217, 338/32 R,

, 338/32 A 51 Int. Cl 1344a 1/18, C23b 5/50 [58] Field' of Search 17/212,,338/32 R, 32 A [56,] v References Cited UNITED STATES PATENTS 2,894,234 3,281,749 10/1966 Weiss 338/3211 In OR Cu.

7/1959 I Weiss et al 338/32 A I 3,490,070 l/l970 Hini 338/32 A Primary Examiner-Mayer Weinblatt Assistant Examiner -Michael F. E'sposito Attorney, Agent, or Firm-Richard S. Sciascia; Joseph M. St. Amand 57 AfisTRAcT A thin-film raster pattern magnetoresistor device produced from an electron beam-recrystallized lnSb film employing a photolithographic process to etch out of the lnSb film a suitable pattern on which micron-size indium or copper stripes are later etched from a superposed metallic film applied on the lnSb by vacuum deposition. The thin-film raster pattern magnetore- I sistor device is a resistor, whose initial resistance in zero magnetic field is between. 10 and 1,000 ohms, and whose resistance increases with an applied magnetic field, reaching a value greater than a factor of 10 on the initial resistance in a magnetic field of 10 kOe.

3 Claims, 2 Drawing Figures InSb GLASS OR NONCONDUCTING ME TAL. OXIDE SUBSTRATE PAIENIEUBEB 31914.

Fig. I.

In OR Cu InSb GLASS 0R NONCONDUCTING METAL OXIDE SUBSTRATE MAGNETORESISTANCE (AR/R I koe 72.4 ,um LINES LENGTH T0 WIDTH RATIO (fl/w) This invention is concerned with the specific electrical and galvanomagnetic properties of raster patternthin film magnetoresistors.

A magnetoresistor is a two-terminal device whose resistance is a function of the transverse magnetic field. Its resistance can be varied slowly or fast, synchronously or asynchronously by varying the magnetic field of an electromagnet acting upon it.

Raster pattern magnetoresistors employing a bar of bulk-crystalline indium antimonide have several shortcomings: (a) while the magnetic sensitivity is very large, the zero field resistance is quite small, i.e., of the order I to ID ohms. This is due to the difficulty of grinding a specimen to the small dimensions needed without breaking it; (b) it is difficult to reduce the size of bulk magnetoresistors to microscopic dimensions. This is desirable in order to focus the magnetic flux generated by a permanent magnet or an electromagnet by means of suitably shaped ferromagnetic pole pieces which concentrate the magnetic flux onto a small area from a broader'source, i.e., I BA where D is the magnetic flux, A is the cross-sectional area of the tip of the flux concentrator and the-magnetic induction B ul-I where p. is the permeability of the flux concentrater; (c) it is difficult to fabricate complex shapes of magnetoresistors, such as for providing different changes in existance for the same amount .of mechanical displacement of a permanent magnet acting on the magnetoresistor. This is desirable if the magnetoresistor is to act as a transducer of complex mechanical motion in two dimensions; (d) afurther disadvantage of bulk raster pattern magnetoresistors is the effect of the electrodes. In arelatively thick device employing bulk indium antimonide the electric current stream lines do not effectively short the Hall field due to the fact that the current stream lines are not everywhere perpendicular to the magneticfiux lines. In the thinfilm magnetoresistor the situation is improved considerably; (e) a further disadvantage of bulk indium antimonide magnetoresistors is thatthe dissipation of heat by thermal conduction is a serious problem due to high temperature coefficientof resistance of lnSb. .In thin film magnetoresistors this large surface to volume ratio provides an effective method for best dissipation and consequently a Smaller rise in temperature .gives the same environmental conditions and the same input current, in comparison with'bulk'lnSb magnetoresistors.

These disadvantages can be overcome by the thin film magnetoresistors of this invention.

The purpose of the invention is the production of a resistor whose value is a function of the applied magnetic field, i.e., magnetoresistors. Materials best suited for the construction of magnetoresistors are the intermetallic compounds InSb and lnAs. In order to produce high sensitivity magnetoresistors, i.e., devices whose resistance shows a strong variation with-magnetic field, the Hall voltage generated in the semiconducting material (InSb or InAs) must be short ci'r cuited. This can be accomplished by an actual physical short circuit of the electrodes, such as in a Corbino disc, with the electrodes disposed in a coaxial fashion, or alternatively, conductive lines applied to a rectangular device can be used to produce an electrostatic short circuit. Still another procedure is the introduction of metallic inclusions into the semiconductor; however, for best results these inclusions should be arranged in a highly directional manner and should have a needlelike shape. This orientation should be normal to the applied magnetic fields and current. Magnetoresistors made of bulk InSb have a higher magnetoresistance (AR/R0) than thin film magnetoresistors. However, their effective resistance R(I-l) is small, at most a few hundred ohms. In many applications it is desirable to have the effective resistance R(l-l) ofthe order of thousands of ohms. This can best be done by means of thin film magnetoresistors.

Thin film magnetoresistors made of the two-phase system InSb and In, i.e., InSb with In inclusions, have a relatively high magnetoresistance. Itis difficult, however, to control their impurity concentration. The latter determines the ultimate temperature sensitivity of the devices. It is also difficult to produce reproducibly the same effective resistance R(0) and magnetoresistance (AR/R0).

The advantages of the present invention are: Higher quality InSb recrystallized by an electron beam microzone process can be produced under reproducible and controlled conditions (Donor impurities can be introduced into the film in accordance with copending U.S. Pat. application, Ser. No. 747,511 filedJuly 25, 1968 by Harry H. Wieder and Arthur R. Clawson for Sulfur Doped Recrystallized InSb Films) now U.S. Pat. No. 3,591,429; A metallic high density, line-raster pattern can be used to cover the films in order to obtain a specific desired magnetoresistance ratio in terms of the length to width ratio of the InSb segment included between two raster lines; The effective magnetoresistance (AR/R0) is that of the (near perfectly) short circuited InSb segment and can be used as a design criterion for various magnetoresistor configurations on either fiat or curved substrates.

, Other objects and many of the attendant advantages of this invention will become readily appreciated as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings wherein:

FIG. 1 is an illustration of a typical magnetoresistor made of electron beam microzone-crystallized InSb film covered by an In or Cu film raster pattern.

FIG 2 shows magnetoresistance (AR/R0) as a function of the magnetic field H for various length to width (l/w) ratios of raster pattern lines.

Thegalvanomagnetic properties of 3 to 5 pm thick InSb films grown by electron beam microzone synthesis and crystallization of composite vacuum deposited (In Sb) layers are essentially the same as those of bulk InSb with a'comparable impurity concentration. The magnetic field and geometry dependence of the magnetoresistance (AR/R0), of raster plate magnetoresistor configurations made of such films have been investigated. A large transverse (AR/R0) is to be expected froma series-connected array of identical InSb parallelipipeds whose thickness is slight with respect to their other dimensions and whose main surfaces'are covered by metallic electrodes. A somewhat smaller (AR/R0) crystallized InSb films deposited on glass or pyrex substrates. A raster pattern of identical lines is' then etched out of the Infilm by means of an etchant which attacks preferentially the In film but not the underlying InSb layer (a suitable etching solution is H PO l-l O in a 40:1 ratio held at a temperature of +80C.) The magnetic field and geometry dependence of (AR/R) of such raster plates is similar to that of single crystal InSb dendrites; magnetoresistance (AR/R0) is proportional to the square of the magnetic field, H up to 2 kOe and is linear in magnetic field H above 3 kOe. It increases with the electron mobility, p. and with the increasing density of the In lines, i. e., with a decrease in the (l/w) ratio.

FIG. 2 shows that the magnetoresistance (AR/R0) of raster pattern magnetoresistors such as shown in FIG. 1 is a linear function of lengthto width (l/w) ratios of raster patternlines for (l/w) 0.2, T 296K and H ID kOe. Width of the metallic lines is identical to spacing between lines, The thickness d and the galvanomagnetic coefficients of the respective InSb films are: 42.4 um film, d 4.05 pm, n 6 X 10 cm', p. 4.7 1()" cm /Vs and p 21.21 X 10' ohm-crr'u- 127 um film, d= 3.06 um'jn 24 .1 0*cm',p.,,= 4.2 X l0 cm /Vs andp 6.19 X 10 3 ohm-cm; 7.2.4 um film, d 1.02 ,m, n 5.2 x 10 cm-12p, 3.3 x '10 em /Vs and p 3.64 X it) ohm-cm. Average values of (AR/R0) and standard deviations are shown for nine specimens of 42.4 fum raster magnetores'istors. Triangular points on ordinate represent Corbino disc magnetoresistance measured in l-I'= l0 kOe. Corbino disc configurations, made of InSb films with the same thickness, electron mobility and donor concentration as those of corresponding raster pattern magnetoresistors, are shown in FIG. 2 to have'(AR/Ro) in fair agreement with the extrapolated (AR/R0) vs (I/w)- function to (l/w) 0. In contrast with two-phase (InSb In) .ordered dendritic film magnetoresistors which exhibit an anomally inthe temperature dependence of (AR/R0) because of the anomalous temperature dependence of M, raster pattern magnetoresistors have been found to be independent of temperature between 296K and 77K to within 2.3 percent.

An advantage of film over bulk magnetoresistors is their high resistance per. film surface areaQSmaIl' surface areas are desirable in order to obtain a high effective magnetic induction, by focusing an applied. mag

netic flux, bymeans of flux concentrators,.onto a magnetoresistor. The large surface-to-volume ratio also provides for an efficient thermal dissipation of Joule heat from the body of a magnetoresistor to its surroundings. In contrast with bulk devices, film magnetoresistors can be processed into complex shapes oneither plane or curved substrates. They can also be made in non-inductive configurations such as required for high frequency applications.

Exact theoretical solutions for the geometrydependent transverse (AR/R0) have been developed they are based on a conformal representation of the electric difference equations to solve Laplaces equation in two dimensions. In each case, it is assumed that the medium is homogeneous and isotropic, that (in H 0) the current streamlines emerge normal tq the equipotential electrodes, that the constraints VXE F 0, VJ O and V I are applicable, and that the resistivity acquires a gyrotropic character in an applied magnetic fieId Here E is the electric field, I the scalar potential and J the current density vector.

Considering a raster plate in a'Cartesian coordinate system, the indium electrodes are in the xy-plane, parallel .to the InSbfilm'surfaces. The current density vector has J, as well as J and J components. The electric field distribution in the presence of a magnetic field where 0 u I-I, is thel-Iall angle. A charge density 0',

is present such that i I a. tanma i/aziyw Only if a 0 is the two-dimensional solution of Laplaces equation applicable.

The results of a resistance-paper analog plot made of (AR/R0), provided that e l. FIG. 2 shows that for a for (l/w) 0.2 is:

the function (w/l)'g(l/w) 11; thus for magnetic field H 10 0a. (AR/R0) 11.5. The small discrepancy between the calculated and measured (AR/R0) is not unreasonable in view of the factthat (2d/l) z 0.19. Similar results have also been obtained on the other magnetoresistors. I-Iigh'raster line densities are desirable in order to decrease the (l/w) ratio and thus to increase the magnetic field sensitivity, 8(AR/Ro)/8I-I of magnetoi'esistors, as well as for increasing their effective resistance per unit surface area. A raster consisting of 1 pm Inllines'is well within the resolution attainable at the present time by means of standard photolithographic techniques. However, the distortion of the equipotential lines can be significant and lead to a reduction in (AR/R0) unless the film thickness dis also reduced. A reduction in film thickness to less than I um does not appear desirable since there is a sharp reduction in ,u with d which may be due to scattering of electrons from the film surfaces.

The photolithographic process used to produce rastel-pattern lnSb film magnetoresistors consists of several steps as follows: First, a chemical photoresist coating (e.g., AZ -l350 Shipley Co. lnc., Los Angeles, Calif.) is applied to a previously processed InSb film, on a glass substrate, grown by electron beam microzone synthesis and crystallizationi'ljhen a particular magnetoresistor pattern mask is superposed on "the coated film and the slide assembly is exposed to ultraviolet light. The exposed photoresist is then removed by the use of a developer (e.g. Developer for'AZ-l350 Photoresist). Chemsol D or aqua regia is then used to etch the pattern out of the InSb film, and, then the remaining photoresist is removed with acetone. Following this an indium film 0.5 to 2 pm thick is evaporated onto the entire glass substrate, including the processed magnetoresistor pattern. A photoresist coating, the same as in the first step, is again applied to the entire slide assembly. Now a raster line pattern mask is super posed on the substrate and exposed to ultraviolet light, and they exposed photoresist is then removed using a developer as aforementioned. The line pattern is then etched out by immersing the slideassembly for 2 to 5 minutes, into a 40:1 solution, for example, of H PO- ,,,H O held at a temperature of 80C;and the remaining photoresist is removed with acetone. The indium film does not adhere to the bare glass in the hot etching solution just described; nor does the etchant attack the InSb film, only the exposed ln lines of the In raster pattern. Thus a raster 'of In lines is left 'on the InSb magnetoresistor pattern'(FIG. 1.). v

The glass substrate surrounding the magnetoresistor pattern is cut away, for example, by means'o f a stream of abrasive particles propelled by'a nitrogen jet, such as an 8.8. White Airbrasive Unit. Electrodes of the magnetoresistor can be Copper electroplated and leads can be attached by solderingorby the use of a conductive epoxy cement. I I

Other semiconducting films such as lnAs or ternary compound films such as lnSb As can be produced by chemical vapor phase transport procedures.

Obviously many modifications andvariations of the b. an electron beam microzone-crystallized semiconducting thin film having microscopic thickness of from 3 to 5 pm and selected from the group' of intermetallic compounds consisting of InSb and lnAs in a magnetoresistor pattern formed on said substrate;

c. a high density raster pattern of identical metallic lines of microscopic thickness on the surface of said conducting thin film, said raster pattern of metallic lines together with said semiconducting thin film providing a series-connected array of identical semiconductor parallelipipeds whose thickness is slight with respect to their other dimensions such that the semiconducting film between the metallic lines has a small length to width ratio in the direction of applied current and the width of the metallic lines is identical with the spacing between said lines;

d. said metallic lines being of the order of 0.5 to 2.0

pm in thickness,

e. the initial resistance of said magnetoresistance device being between 10 and 1,000 ohms which resistance increases with an applied magnetic field reaching a' value greater than a factor of 10 of the initial'resistance in a magnetic field of i0 kOe, said magnetoresistor also having a large surface-tovolume ratio which provides for efficient thermal dissipation of heat from the body of said magnetoresistor to its surroundings.

2. A high sensitivity magnetoresistor device as in claim. 1-wherein said metallic lines are In. I

3. A high sensitivity magnetoresistor device as in 

1. A HIGH SENSITIVITY RASTER PATTERN MAGNETORESISTOR DEVICE WHOSE RESISTANCE SHOWS A STRONG VARIATION WITH MAGNETIC FIELD, COMPRISING: A. A NON-CONDUCTING SUBSTRATE, B. AN ELECTRON BEAM MICROZONE-CRYSTALLIZED SEMICONDUCTING THIN FILM HAVING MICROSCOPIC THICKNESS OF FROM 3 TO 5 UM AND SELECTED FROM HE GROUP OF INTERMETALLIC COMPOUNDS CONSISTING OF INSB AND INAS IN A MAGNETORESISTOR PATTERN FORMED ON SAID SUBSTRATE; C. A HIGH DENSITY RASTER PATTERN OF IDENTICAL METALLIC LINES OF MICROSCOPIC THICKNESS ON THE SURFACE OF SAID CONDUCTIG THIN FILM SAID RASTER PATTERN OF METALLIC LINES TOGETHER WITH SAID SEMICONDUCTING THIN FILM PROVIDING A SERIESCONNECTED ARRAY OF IDENTICAL SEMICONDUCTOR PARALLELIPIDEDS WHOSE THICKNESS IS SLIGHT WITH RESPECT TO THEIR OTHER DIEMSIONS SUCH THAT THE SEMICONDUCTING FILM BETWEEN THE METALLIC LINES HAS A SMALL LENGTH TO WIDTH RATIO IN THE DIRECTION OF APPLIED CURRENT AND THE WIDTH OF THE METALLIC LINES IS IDENTICAL WITH THE SPACING BETWEEN SAID LINES; D. SAID METALLIC LINES BEING OF THE ORDER OF 0.5 TO 2.0 UM IN THICKNESS; E. THE INTIAL RESISTANCE OF SAID MAGNETORESISTANCE DEVICE BEING BETWEEN 10 AND 1,000 OHMS WHICH RESISTANCE INCREASES WITH AN APPLIED MAGNETIC FIELD REACHING A VALUE GREATER THAN A FACTOR OF 10 OF THE INITIAL RESISTANCE IN A MAGNETIC FIELD OF 10 KOE, SAID MAGNETORESISTOR ALSO HAVING A LARGE SURFACE-TO-VOLUME RATIO WHICH PROVIDES FOR EFFICIENT THERMAL DISSIPATION OF HEAT FROM THE BODY OF SAID MAGNETORESISTOR TO ITS SURROUNDINGS.
 2. A high sensitivity magnetoresistor device as in claim 1 wherein said metallic lines are In.
 3. A high sensitivity magnetoresistor device as in claim 1 wherein said metallic lines are Cu. 